fix(artifact-caching-proxy) set defaults on PDB

[dduportal]

This PR fixes the error seen in jenkins-infra/kubernetes-management#4460 (review)

[ERROR] templates/: template: artifact-caching-proxy/templates/pdb.yaml:9:18: executing "artifact-caching-proxy/templates/pdb.yaml" at <.Values.poddisruptionbudget.minAvailable>: nil pointer evaluating interface {}.minAvailable

💡Notes:

• Unit test added to cover the case when a PDB is created because multiple replicas are set up but with its default setup. This unit tests returns the error above with the version 0.15.0 of this chart
• Unit test added to verify the coverage of custom value maxUnavailable

[dduportal]

Info: A diff on production with this fix shows the following:

+ # Source: artifact-caching-proxy/templates/pdb.yaml
+ apiVersion: policy/v1
+ kind: PodDisruptionBudget
+ metadata:
+   name: artifact-caching-proxy
+   labels:
+     helm.sh/chart: artifact-caching-proxy-0.15.1
+     app.kubernetes.io/name: artifact-caching-proxy
+     app.kubernetes.io/instance: artifact-caching-proxy
+     app.kubernetes.io/managed-by: Helm
+ spec:
+   minAvailable: 1
+   selector:
+     matchLabels:
+       app.kubernetes.io/name: artifact-caching-proxy
+       app.kubernetes.io/instance: artifact-caching-proxy

which looks good to me!